1. Field of the Invention
The present invention relates to an electron beam lithography apparatus and an electron beam lithography method. In particular, the present invention relates to an electron beam lithography apparatus and an electron beam lithography method which are capable of achieving a higher drawing speed while ensuring drawing accuracy by adjusting an electron beam in accordance with the degree of importance of a device pattern to be formed on a sample.
2. Description of the Prior Art
For the purpose of improving throughput, an electron beam lithography apparatus is provided with a variable rectangular opening or a plurality of stencil mask patterns in a stencil mask, and transfers a desired pattern onto a wafer through exposure by selectively using the opening or patterns with beam deflection.
For example, Japanese Patent Application Publication No. 2004-88071 discloses an electron beam exposure apparatus for character projection lithography as such a lithography apparatus. The character projection lithography is performed as follows. Firstly, a beam is applied to one pattern region, e.g., a 300 μm×300 μm region, selected by beam deflection from a plurality of stencil patterns, e.g., 100 stencil patterns, disposed on a mask to shape a beam cross section into the shape of a stencil pattern. The beam which has passed through the mask is deflected and bent back by a downstream deflector, reduced at a certain reduction ratio, e.g., 1/10, which is determined depending on the electrooptic system, and transferred onto the surface of a sample. When stencil patterns on the mask are appropriately prepared in accordance with a device pattern to be transferred, the number of necessary exposure shots is greatly reduced, and the throughput is improved, in comparison with the case where only a variable rectangular opening is provided.
Further, there has been proposed a multi-column electron beam exposure apparatus collectively including multiple columns (hereinafter referred to as column cells) each of which is a small-sized column of an exposure apparatus such as described above, and which are arranged over a wafer to perform exposure in parallel (can be referred to, T. Haraguchi et al. J. Vac. Sci. Technol, 2004, B22, 985). Each of the column cells is equivalent to a column of a single-column electron beam exposure apparatus, and the multi-column apparatus as a whole multiplies an exposure throughput by the number of columns because of parallel processing.
When exposure is performed, the deflection position of an electron beam is changed by a deflector. At this time, since the deflector is given a signal corresponding to a deflection amount, there is a settlement wait time until the electron beam settles at a deflected position.
In general, an exposure region is divided into multiple subfields. A deflection amount is fixed such that a deflection position by a main deflector is around the center of one subfield. A voltage corresponding to exposure data is applied to a sub-deflector. Thus, a pattern within the subfield is drawn. Exposure is performed at high speed by reducing the frequency of deflections by the main deflector, which requires a long settlement wait time, as described above.
As a technique for reducing a settlement wait time, for example, Japanese Patent Application Publication No. 2004-72123 describes a technique of reducing an apparent settlement wait time to approximately 3 μs in deflection by a main deflector by detecting a difference between main deflection data and a voltage actually applied to the main deflector, and applying this difference to a feedback deflector.
As described above, the throughput of drawing is improved by using character projection lithography or a variable rectangular beam. The throughput can be further improved by using multiple columns at the same time.
Moreover, even when there is a settlement wait time after electron beam deflection, the overall throughput of processing can also be improved by increasing the size of a beam to be applied using character projection lithography or a variable rectangular beam.
However, as the size of the beam increases, the scattering of electrons in the beam increases due to Coulomb effects, and the difficulty of high-accuracy drawing increases. Accordingly, a drawing pattern requiring high accuracy must be drawn with the size of the beam reduced. Thus, it is difficult to improve processing speed.
Further, there are device patterns for various uses such as gate portions of transistors and interconnections. Respective accuracies required for device patterns are not necessarily uniformly equal to each other. If all device patterns are drawn with an accuracy for device patterns requiring high accuracy, high-accuracy drawing can be performed overall, but, on the other hand, it will become difficult to improve processing speed.